1. Field of the Invention
This invention relates to the coding of video signals and in particular the coding of video signals for storage and subsequent transmission.
2. Related Art
Broadcast quality television signals require around 6 MHz of analogue bandwidth or in excess of 100 Mbit/s of information for a digital format obtained by sequentially sampling an analogue signal to produce a PCM digital signal.
Such high bit rate signals are expensive to transmit and for transmission cost reasons therefore it is desirable to reduce the amount of information required. This can be done by taking advantage of the correlation between neighbouring elements of a picture (pixels) and thus compromising between the reduction in information and the quality of the picture.
Redundancy reduction techniques assume there is some correlation between neighbouring pixels, either in space and/or in time. For instance, in an area of a scene which is relatively uniform (for instance a wall of a room), the pixel values of neighbouring pixels within this area are likely to be fairly close. Similarly, in a fairly static scene, the pixels of one frame will correspond closely to the equivalent pixels of a previous frame.
Hence pixels of a single frame can be coded with respect to their relationship to each other (intraframe coding) and/or with respect to their relationship with pixels of neighbouring frames (interframe coding). Intraframe coded frames (intrapictures) can clearly be decoded without reference to any other frame whilst interframe coded frames (interframes) require information relating to the frames used in the prediction. Differential coding techniques may also be used to compress video signals further. Since interframe differential coding may result in the irretrievable loss of some information owing to transmission errors, artifacts will occur in a decoded picture if only interframe differential coding is used. It is thus usual for a combination of intra- and inter-frame coding techniques to be used, the intraframes restoring the integrity of the decoded signal.
Other compression techniques can also be employed; for instance transform coding which seeks to exploit the correlation of pixel magnitudes within a frame by finding another set of coefficients, the magnitude of many of which will be relatively small. These coefficients can then be quantised coarsely or omitted altogether. The transform coefficients of a frame can thus be coded using less information. One popular form of transform coding uses the discrete cosine transform (DCT).
Another form of interframe compression technique is motion compensation coding which involves the identification of areas in successive frames which appear to correspond but have moved within the frame. A motion vector is calculated for each such area and a predicted frame is then formed from the previous frame and the motion vectors. Errors between the predicted frame and the actual frame are then calculated and, together with the motion vectors, coded. This may result in less information than that of two frames taken together.
The compression of video signals is the subject of much standardisation work. One such standard is the ISO-IEC 11172 standard "Coding of moving pictures and audio for digital storage media at up to about 1.5 Mbit/s", known as MPEG-1, which relates to the storage of video and associated audio on digital storage media such as CD-ROM, digital audio tape (DAT), tape drives, writable optical drives or for transmission over telecommunication channels such as an integrated services digital network (ISDN) and local area networks. Such coding techniques are attractive for the provision of audiovisual services over limited bandwidth systems.
The time taken to access and retrieve a stored video signal can be prohibitive to the provision of interactive video services in which a consumer selects a particular service from a range of available services. The access time is increased dramatically if the stored video signal requires further processing before it can be output to a display device.
A recent development in such services is the provision of home entertainment or shopping services in which a consumer selects a service from a range on offer and the relevant video signal is transmitted to the consumer's premises from a central server. In a video-on-demand environment, for example, a consumer uses a central video server in the manner of a remote video cassette player. Consumers therefore expect the same facilities as they would have on their own video cassette player e.g. the facility to play, pause, stop, fast forward and reverse.
Various processors are available which provide these facilities. When a consumer requests play, the coded video signal stored at the remote server is transmitted to the consumer. A local decoder at the consumer's premises decodes the incoming signal to produce a video image on a television set. In the pause mode, a pause signal is sent to the server which, in response, sends a signal to the consumer's decoder indicating that the frame is unchanged.
When fast forward or reverse is selected however, the coded signal must be processed further by the video server. When a consumer requests fast forward, a signal is sent to the server which then transmits every, say, fourth frame of the coded signal. If the video signal is in an uncompressed format, the server has to locate the beginning of every fourth frame in the video signal and transmit these to the consumer. This is very processor and time intensive and may result in a delay that would be unacceptable to consumers.
Similarly, if compression coding techniques have been employed, the fifth frame of the picture may have been coded with reference to the fourth frame. If in the fast forward mode only the first, fifth, ninth etc. frames are to be sent, each frame to be sent must be recoded with respect to the preceding frame to be sent. This is very processor- and time-intensive. For video signals coded using only intraframe coding, it is known to provide a fast forward mode by extracting the intraframe coded frames (intrapictures) from the encoded video signal and transmitting these frames in their original order. Similarly they could be sent in the reverse order for the fast reverse mode. However, not only does the server, on receiving a fast forward request signal, have to search the coded signal for intrapictures but the bit rate of the resulting signal will be increased as compared to the play mode since the intrapictures include very little compression. The decoder at the consumer's premises therefore has to be able to manage excessive changes in the bit rate.